Exploring the limits of in-situ U-Pb dating of metamorphic garnet

Recent advances in analytical techniques and instrumentation allow for the analysis of increasingly smaller sample volumes and lower concentrations. This development significantly expands the possibilities of in-situ geochronology, e.g., LA-MC-ICPMS. Minerals with low U (and Pb) contents such as garnet become the target of in-situ U-Pb geochronology since ages can potentially be obtained from single (sub-)mm-sized garnet grains in thin sections. In this contribution, we explore the current limits of in-situ U-Pb geochronology: what are the minimum concentrations from which an accurate and precise U-Pb age can be obtained?

For that purpose, we have analysed garnets from three different localities that were unsuccessfully analysed in the past using a single-collector sector-field Element XR instrument at FIERCE. These garnets have been re-analysed at FIERCE using a Neptune Plus MC-ICPMS coupled to a RESOLution-LR ArF Excimer laser. The analyses were performed in static mode measuring the masses ²⁰⁶Pb and ²⁰⁷Pb with Secondary Electron Multiplier (SEM) and ²⁰²Hg, ²⁰⁴Pb and ²³⁸U with the Multiple Ion Counters (MIC). With a spot diameter of 193 μm (round) and a fluence of 2 J/cm² at 15 Hz, ca. 18 µm pit depth was ablated in 18s analysis time, resulting in a total of 2 µg of ablated material. This is more than 2,000 times less material compared to conventional isotope dilution analyses and 3,000 times less U than for a typical LA-ICPMS zircon analysis (20 µm spot). Although the analysed garnets typically have U contents below 10 ng/g, about 15–30 spots are commonly sufficient to define a regression line in the Tera-Wasserburg diagram, yielding a precision of typically <3 % for the lower intercept age. Challenges and details of the method will be discussed using samples of metamorphic garnet from Kaapvaal craton granulites and Eastern and Western Variscan eclogites.